Fossil fuel consumption increases 2% a year due to transportation fuels and specialty chemicals for plastics and synthetic fibers such as p-xylene, a monomer of polyethylene terephthalate. p-Xylene demand was over 50 million tons in 2021 and will increase by 5% a year through 2026. Therefore, sustainable p-xylene production is desired. p-Xylene is produced renewably through Diels-Alder cycloaddition of biomassderived 2,5-dimethylfuran (DMF) with ethylene from bio-ethanol and dehydration over an acid catalyst. Industrial aluminosilicate zeolite catalysts achieve a selectivity of 75%, with loss to side products and coking. A new class of catalysts, phosphoric acid-containing aluminum-free zeolites, P-zeosils, like dealuminated zeolite P-BEA show high selectivity to p-xylene over 95%. However, the active site structure and structure-property relationships leading to the high selectivity are unknown. Herein, the catalytic performance of homogeneous phosphoric acid in p-xylene production with and without adding siliceous zeolite in the reactions is investigated. Phosphoric acid is active for p- vii xylene production, but phosphoric acid adsorbing on the siliceous zeolites during the reaction shows an improved selectivity. However, impregnation of phosphoric acid on the siliceous zeolite before the reaction forming the P-zeosil achieves the highest selectivity (>95%). Changing Si/P ratio and calcination temperature on the impregnated P-zeosils are investigated through X-ray diffraction (XRD), nitrogen adsorption and 31P solid state magic angle spinning nuclear magnetic resonance (MAS NMR). The P-zeosils exhibit Brønsted acid sites (BAS). Good agreement was found between DMF initial reaction rate and the BAS density. Low Si/P ratio (Si/P